This invention relates to Megarad Dose Meters and more particularly to a Potentiometric Chemical Dosimeter.
Ionizing radiations are being used in the agricultural, biological, chemical, food, medical, pharmaceutical and allied industries for disinfestation, preservation and sterilization of raw and finished products and for the manufacture of chemicals and polymers. For many of these applications the dose of radiation required is in the range 0.2 to 50 megarad. One important industrial process is the radiation sterilization of medical supplies, for which doses in the range 2.5 to 4.5 megarads have been specified by Health Authorities throughout the world. Other important processes are polymerization and cross-linking which require radiation doses of about 1 and 10 megarads respectively.
Numerous physical and chemical methods have been developed for the measurement of dose in the megarad range. Only a few are convenient to use and sufficiently reliable to be widely accepted and useful for routine industrial applications. Chemical methods rely on colorimetric change in liquids and solids as a measure of radiation dose, the liquids include ceric sulphate solution and ferrous-cupric sulphate solution and some solid materials which are used include plastics such as polymethylmethacrylate and polyvinyl chloride. While plastic dosimeters are convenient they are not stable with time and require strict standardization in use as well as regular calibration against a reference dosimeter, for example the widely accepted ferrous sulphate chemical dosimeter.
However, of the many methods for dosimetry, the requirements for a conveniently useful dosimeter are rarely achieved. The criteria which are commonly not met are:
1. In chemical dosimeters the response should bear a known fixed relationship to primary radiolytic yields.
2. The dosimeter should be stable for an indefinite period before and after irradiation.
3. The responses should be linear over a wide dose range.
4. The effect of temperature on the response of the dosimeter during and after irradiation should be known and reproducible.
5. The dosimeter should be easily prepared.
The ceric dosimeter system which is a solution of ceric sulphate in sulphuric acid has been investigated for more than a decade by many research workers and is widely accepted. In fact, the International Commission on Radiological Units and Measurements rated this system as the next most useful to the internationally accepted ferrous sulphate dosimeter and recommended its use as a reference standard for radiation doses exceeding 0.05 megarads.
However, the ceric sulphate dosimeter suffers from two disabilities. Firstly, the reduction is sensitive to impurities, particularly at low ceric ion concentrations (&lt;0.001 M) and therefore there is a stringent requirement on the purity of solutions and cleanliness of apparatus. Secondly, the ceric concentration is measured by spectrophotometry or titration. Neither method is well suited to routine use since the spectrophotometric method requires a dilution to a concentration at which Beer's Law is obeyed and the titration is time consuming and requires skill on the part of the operator.
Following chemical analysis of the dosimeter the dose must be calculated or read from specially prepared tables or graphs for converting the measurement to dose. No apparatus apart from that which we claim as our invention allows the direct reading of dose from the ceric dosimeter system unaltered by dilution or titration.